1. Field of the Invention
The invention is directed to a device for indirectly heating fluids, particularly for high temperature processes. The device includes a heating space in which at least one tube coil is arranged. The tube coil is constructed in a planar member and the fluid to be heated can be conducted through the tube coil. Radiation heat of a heat radiator may act from the outside on the tube coil.
2. Description of the Related Art
Such devices are required particularly for carrying out high temperature processes which occur frequently in oil refining and petrochemistry. The fluid to be heated, e.g. liquid or gaseous hydrocarbons or a mixture of hydrocarbons and steam, is conventionally guided through a heating space in heat exchanger tubes and heated by the tube wall of the heat exchanger tubes without coming into direct contact with the heating medium. The transfer of heat to the tube wall is usually primarily effected by heat radiation which proceeds from an open flame of a combustible material burned in the heating space and to a small extent by the hot combustion gases by way of convection. The heat exchanger tubes run through the heating space in the form of tube coils.
The great disadvantage of open flames is that it is very difficult to adjust a desired geometric form of the flame and a temperature distribution which is as uniform as possible. Uniform heating ratios are therefore very difficult to achieve particularly under variable operating conditions. The boundaries for corresponding intervention for purposes of control are very narrow in practice. Changes in the flame geometry are equivalent to changes in the distance of individual locations of the heat exchanger tubes from the "flame surface". This means that the flow of heat through the heat exchanger tubes always fluctuates considerably not only along the tube coil. In particular, a nonuniform flow of heat can also be determined along the circumference of the heat exchanger tubes, since the individual partial pieces of the tube surface differ in their alignment with respect to the flame in a compulsory manner and are sometimes even remote from the flame and accordingly irradiated at different intensities. This can lead to localized overheating at isolated points of the heat exchanger tubes and simultaneously to a considerable drop below the desired tube wall temperature at other locations. Accordingly, thermal damage to the heat exchanger tubes can occur proceeding from the outside on the one hand and undesirable effects can also be triggered with respect to the fluid to be heated (e.g. coking of the inner surface of the tubes) on the other hand. In conventional furnaces for high temperature processes, the differences are often so great that the ratio of the maximum to mean heat flow in the walls of the heat exchanger tubes can lie in the range of 3:1 to 4:1.
It is known in practice to burn gaseous combustibles (gas or evaporated liquid combustibles) without flame formation in a burner with a heat radiation surface in that the gaseous combustible which is mixed with an oxygen containing gas (e.g. air) is guided through a porous radiation body and ignited and burned on its outer surface. The ignition is effected by the glowing of this outer surface (heat radiation surface). Corresponding to the geometric form of the radiation body, the heat radiation surface has a regular shape which, in contrast to an open flame, does not change when the supply of combustible material changes. Moreover, the temperature distribution within the heat radiation surface is very uniform.
Such a burner with heat radiation surface (heat radiator) is known e.g. from U.S. Pat. No. 4,722,681. Its radiation body is formed from a ceramic fiber matrix and has great length and width compared to the physical depth of the burner, resulting in a large heat radiation surface. This burner is provided for thermally treating long webs of paper or woven materials.
Further, it is known from U.S. Pat. No. 4,865,543 to use a burner with a heat radiation surface for heating an apparatus, a flat tube coil being guided as heat exchanger through its heating space. The fluid to be treated flows in the tube coil and is heated indirectly as a result of the heat radiation. As a result of the combustion, the heat radiator which is constructed as a fiber burner and arranged at the base of the heating space releases hot combustion gases which rise up and are carried out of the heating space at the top. The tube coil of the heat exchanger lies in a vertical plane and the tubes of the individual loops of the tube coil are arranged substantially horizontally.
Finally, a heating apparatus is known from EP 0 385 963 A1 which is formed from a cylindrical housing in which a likewise cylindrical ceramic hollow body with porous walls is arranged. Moreover, another cylindrical heat exchanger is installed in the housing at a distance from the cylindrical surface of the ceramic body, a heat carrier medium flowing through this cylindrical heat exchanger. A mixture of gaseous combustible material and an oxygen containing gas at above-atmospheric pressure is introduced in the intermediate space between the casing of the housing and the outer surface of the ceramic body. This mixture flows through the ceramic body and is burned when ignited on the inner surface of the ceramic body. The hot flue gases occurring as a result of the combustion can enter the hollow space enclosed by the heat exchanger through suitable through-openings in the outer surface area of the cylindrical heat exchanger while giving off heat and can be carried off from there to the outside. This heating apparatus in which a large portion of the heat absorbed by the heat exchanger is transmitted by convection is primarily conceived as a heating furnace for heating systems in buildings and is not suitable for implementing high temperature processes.
The fluid to be heated is introduced into the heat exchanger from above and drawn off again at the bottom so that the "transporting direction" of the tube coil is directed opposite to the upwardly directed flow of the combustion waste gases. Evaporated liquid combustibles such as kerosene, diesel, naphtha or alcohol are used for the combustion.
In this known apparatus the lower portions of the heat exchanger tube coil are exposed to an intensive heat radiation, while the upper portions can no longer be reached by the heat radiation of the burner and are substantially heated by convection. But the heat radiation can only act on a part of the tube surface even in the lowest heat exchanger tube.
Whereas the lateral regions of the horizontally disposed tubes are irradiated to a considerably lesser extent than the underside of the heat exchanger tube at the bottom, the upper sides of the heat exchanger tubes are not directly irradiated at all. This means that the flow of heat is subject to considerable fluctuations in the circumferential direction of the heat exchanger tubes as well as in the transporting direction of the heat exchanger.
A device for indirectly heating fluids is known from EP 0 233 030 A2, a plurality of rows of flat radiation burners arranged one on top of the other being mounted in its heating space at a distance from one another and so as to be parallel to one another. A tube heat exchanger with a plurality of substantially horizontally extending tube loops arranged substantially in two vertical parallel planes relative to the radiation burners is located in the intermediate spaces of these rows of burners. The intermediate spaces between every two directly adjacent tube loops are open. The distance from the effective radiation surfaces of the radiation burners as well as the irradiation angle of the heat radiation vary as seen along the tube circumference of the tube loops, so that the temperature of the tube wall is nonuniform along the tube circumference.